1. Field of the Invention
The present invention relates to ultra-high density memory based on the principle of scanning tunneling microscope.
2. Related Background Art
In recent years, memory elements are used in the most important areas of electronic industry for producing computers, related devices thereof, video disks, digital audio disks, and so forth. Research and development of the memory elements are actively being conducted. Generally, memory elements are required to satisfy the items below:
(1) High density and large capacity of recording, PA0 (2) High response speed of recording and reproduction, PA0 (3) Small error rate, PA0 (4) Low power consumption, PA0 (5) High productivity and low cost, and so forth.
Heretofore, magnetic memories and semiconductor memories employing a magnetic material or a semiconductor are mainly used for memory devices. However, as the result of recent progress in laser technique, optical memory elements have come to be used which employ inexpensive high-density recording medium utilizing an organic thin film of an organic pigment, a photopolymer, or the like.
On the other hand, in recent years, scanning tunneling microscope (hereinafter referred to as "STM") has been practicalized which enables direct observation of the electronic structure of atoms on a surface of a conductor (G. Binnig, et al.: Helvetica Physica Acta 55, 526 (1982)). This technique enables measurement of real spatial images of materials, whether single crystalline or amorphous, at extremely high resolution with a low electric power without impairing the medium by current. Furthermore, the STM can be operated in the atmosphere. Therefore the STM is promising in a variety of application fields.
STM utilizes a tunnel current which flows, on application of voltage, between a metallic probe and an electroconductive substance placed as close as about 1 nm to each other. This tunnel current is highly sensitive to the change of the distance between the two. Accordingly, real spatial surface structure can be depicted and simultaneously various information on the entire electron cloud of the atoms on the surface can be read by scanning the surface with the probe so as to keep the tunnel current constant. In the observation, the resolution in the plane direction is about 1 .ANG., so that high-density recording and reproducing is achievable in precision of an order of atom size (several .ANG.) by utilizing the principle of STM. In this recording-reproducing method, it is proposed that recording is conducted by changing the surface state of an appropriate recording layer with a high-energy electromagnetic wave such as a corpuscular beam (an electron beam or an ion beam) and an X-ray, or an energy beam such as visible ray and ultraviolet ray, and the reproducing is conducted by STM.
Another method of recording-reproducing is proposed in which a material exhibiting memory effect in current-voltage switching characteristics such as a thin film layer of a .pi.-electronic organic compound, a chalcogenide, or the like is used as the recording layer in which recording and reproducing are conducted by STM (Japanese Patent Application Laid-Open No. Sho-63-161552). In this system, recording is made on a recording layer having electric memory effect by applying voltage directly by means of a probe electrode of STM, by switching the recording layer ideally in a unit size of an atomic order, whereby recording and reproducing is practicable in much higher density than that in optical recording.
The above-described recording-reproducing method requires detection of the position of the probe electrode in XY directions (on a plane of the recording medium) and control of correction of the position (namely, tracking) in order to record and reproduce a large quantity of information practically. In a method proposed, the tracking is conducted with high density and high precision by utilizing the atom arrangement in a substrate of a recording medium.
Separately, for simplification of the tracking, a proposed method is that guiding grooves (tracks) are provided preliminarily by forming projections and recesses on a substrate of a recording medium, and the probe electrode is moved by tracing the recessed portion or the projected portion of the track.
In the latter method, the track can be detected by the probe electrode in two ways: (1) the surface shape is recognized by controlling the distance between the probe electrode and the recording medium by giving feedback to a probe-height-controlling element so as to keep constant the current between the probe electrode and the recording medium in response to the change of the shape of the surface of the recording medium while the current is continuously measured; or (2) the surface shape is recognized by converting the current intensity to the distance between the probe electrode and the recording medium without using the above feedback.
In conventional recording-reproducing methods, particularly in such tracking as mentioned above, the track on the recording medium has been formed by mask vapor deposition or photolithography. In such methods of track formation, the fineness of the track pattern is limited. For example, the possible lower limit of the size thereof is about several ten .mu.m in mask vapor deposition, and several tenth .mu.m in photolithography. Therefore a desired high-density recording could not readily be achieved.
In erasing the recorded portion after practical recording and reproducing, according to the technique of the aforementioned electric memory effect, the probe electrode has to be made to access to the recorded bit and the recorded bit has to be switched to the original state individually. This requires enormous time when a wide range of information, for example, the entire of a recording layer or sector units of information is erased, which requires much time and makes difficult the high-speed response disadvantageously. Furthermore, in such technique, high controllability is required in voltage value and distance between the recording medium and the probe electrode. The deviation from the optimum conditions will cause insufficiency of erasing. Hence a recording medium and an erasing method are desired which do not require high controllability and enable simultaneous erasing of a number of bits.